DHCR24 Knock-Down Induced Tau Hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, Ser396 Epitopes and Inhibition of Autophagy by Overactivation of GSK3β/mTOR Signaling

Abstract

Accumulating evidences supported that knock-down of DHCR24 is linked to the pathological risk factors of AD, suggesting a potential role of DHCR24 in AD pathogenesis. However, the molecular mechanism link between DHCR24 and tauopathy remains unknown. Here, in order to elucidate the relationship between DHCR24 and tauopathy, we will focus on the effect of DHCR24 on the tau hyperphosphorylation at some toxic sites. In present study, we found that DHCR24 knock-down significantly lead to the hyperphosphorylation of tau sites at Thr181, Ser199, Thr231, Ser262, Ser396. Moreover, DHCR24 knock-down also increase the accumulation of p62 protein, simultaneously decreased the ratio of LC3-II/LC3-I and the number of autophagosome compared to the control groups, suggesting the inhibition of autophagy activity. In contrast, DHCR24 knock-in obviously abolished the effect of DHCR24 knock-down on tau hyperphosphrylation and autophagy. In addition, to elucidate the association between DHCR24 and tauopathy, we further showed that the level of plasma membrane cholesterol, lipid raft-anchored protein caveolin-1, and concomitantly total I class PI3-K (p110α), phospho-Akt (Thr308 and Ser473) were significantly decreased, resulting in the disruption of lipid raft/caveola and inhibition of PI3-K/Akt signaling in silencing DHCR24 SH-SY5Y cells compared to control groups. At the same time, DHCR24 knock-down simultaneously decreased the level of phosphorylated GSK3β at Ser9 (inactive form) and increased the level of phosphorylated mTOR at Ser2448 (active form), leading to overactivation of GSK3β and mTOR signaling. On the contrary, DHCR24 knock-in largely increased the level of membrane cholesterol and caveolin-1, suggesting the enhancement of lipid raft/caveola. And synchronously DHCR24 knock-in also abolished the effect of DHCR24 knock-down on the inhibition of PI3-K/Akt signaling as well as the overactivation of GSK3β and mTOR signaling. Collectively, our data strongly supported DHCR24 knock-down lead to tau hyperphosphorylation and the inhibition of autophagy by a lipid raft-dependent PI3-K/Akt-mediated GSK3β and mTOR signaling. Taking together, our results firstly demonstrated that the decrease of plasma membrane cholesterol mediated by DHCR24 deficiency might contribute to the tauopathy in AD and other tauopathies.

The role of DHCR24 in the pathogenesis of AD: re-cognition of the relationship between cholesterol and AD pathogenesis

“…Moreover, in DHCR24 knockout (KO) mice the brain cholesterol lack with age, and brain cholesterol deficiency in 3-week-old was associated with altered membrane composition including disrupted detergent-resistant membrane domain (DRM) structure [ 3 , 71 ]. Similarly, in silencing DHCR24 cell model, it was found that cell desmosterol and 7-dehydrocholesterol (7-DHC) are significantly elevated, and cell cholesterol is greatly decreased [ 9 , 24 , 127 ]. So, this genetic defect manifests that the defect of DHCR24 enzyme activity leads to cholesterol deficiency, suggesting the critical role of DHCR24 in maintaining cholesterol synthesis and homeostasis.…”

“…Thus, the above data suggest that downregulation of DHCR24 is obviously linked to the major risk factors from AD, suggesting a potential causative link between DHCR24 downregulation and major risk factors from AD. Moreover, a growing body of research has shown that deficiency of DHCR24 activity could induce lowering cholesterol level of neuronal cells and disruption of membrane lipid-raft structure and function, leading to the disregulation of cellular cholesterol homeostasis, and abnormality of cell signaling [ 3 , 9 , 24 , 71 , 122 ]. In addition, increasing evidences support that downregulation of DHCR24 could lead to Aβ production, apoptosis of neuronal or glial cells, hyperphosphorylation of microtubule-associated protein tau (tau), inhibition of autopagy, and inflammation, which are tightly associated with AD and other degenerative diseases [ 9 , 24 , 45 , 71 , 83 , 95 , 127 , 155 ].…”

“…Moreover, a growing body of research has shown that deficiency of DHCR24 activity could induce lowering cholesterol level of neuronal cells and disruption of membrane lipid-raft structure and function, leading to the disregulation of cellular cholesterol homeostasis, and abnormality of cell signaling [ 3 , 9 , 24 , 71 , 122 ]. In addition, increasing evidences support that downregulation of DHCR24 could lead to Aβ production, apoptosis of neuronal or glial cells, hyperphosphorylation of microtubule-associated protein tau (tau), inhibition of autopagy, and inflammation, which are tightly associated with AD and other degenerative diseases [ 9 , 24 , 45 , 71 , 83 , 95 , 127 , 155 ]. Therefore, previous studies strongly support that that the reduced cholesterol level in plasma membrane and/or intracellular compartments by the deficiency of DHCR24 obviously contributes to neurodegeneration such as AD (Fig.…”

“…Furthermore, the brain cholesterol deficiency seems to be an early and common driving factor in the onset and development of AD. And the brain cholesterol deficiency could be intimately linked with the generation of β-amyloid, tauopathy, synaptic loss, neuronal apoptosis and death, which are associated with the pathogenesis of AD [ 5 , 9 , 21 , 24 , 45 , 73 – 75 , 93 , 95 , 115 ]. Based on previous data and research on DHCR24, we suppose that the brain cholesterol deficiency/loss could trigger the onset and progression of AD.…”

Agomelatine Prevents Amyloid Plaque Deposition, Tau Phosphorylation, and Neuroinflammation in APP/PS1 Mice

“…Besides, knocking out DHCR24 was reported to aggravate Aβ production and accumulation in the AD mouse model ( Crameri et al, 2006 ), and overexpressing DHCR24 could decrease Aβ levels and inhibit Aβ toxicity in vitro ( Sarajärvi et al, 2009 ). Bai et al (2021) indicated that DHCR24 knock-down induced tau protein hyperphosphorylation in SH-SY5Y cells. Another study showed a low serum DHCR24 protein level in AD patients with diabetes mellitus (DM) ( Önmez et al, 2020 ).…”